In response to ecological concerns, the Federal Environmental Protection Agency (EPA) has developed regulations regarding leakage from underground storage tanks. To assist in the detection of leakage, observation wells and monitoring wells are installed near the tanks. An observation well is a well located within an area around the tank which was excavated during the installation of the tank. A monitoring well is a well that is outside the tank excavation area and extends below the water table.
Observation and monitoring wells have similar construction. The term "observation well" will be used herein to refer to both observation wells and monitoring wells.
Surface water above the tanks contaminated with hydrocarbons, e.g., rainfall mixing with surface spills from filling and emptying the tanks, can enter the well.
Observation wells can be used for detecting leakage other than that from underground storage tanks, for example leakage from landfills.
The first of these designs is shown in the American Petroleum Institute Industry Standards, API Recommended Practice 1615. In this design, a metal manhole with a metal manhole cover is set in concrete in a small raised area on the ground. A 2-12 inch polyvinyl chloride, PVC, pipe is used as the screen, casing or screen for the well and the pipe has 0.020 inch slots horizontally located in the lower two-thirds of the pipe. The pipe is permanently set in the concrete around the manhole. A seal or surface water barrier of consecutive layers of bentonite and sand or gravel are located below the concrete and may extend several feet below the surface. EPA regulations require that the well be sealed to the top of the ground that surrounds the well screen called the filterpack. The bentonite seal extends far into the filterpack. The uppermost slots are thus much below the filterpack rendering the well an inaccurate representation of the conditions of the site to be monitored. Bentonite looses its ability to seal when brought into contact with hydrocarbons.
A second observation well configuration specified by Shell Oil Company is shown in FIG. 1 of the drawings. In this configuration, a manhole with a manhole cover made of metal is set in concrete in a slightly raised mound in the concrete. The slotted, 4-inch PVC, polyvinyl chloride, casing is connected to the manhole by a screen which is connected to the casing by a stainless band clamp and also attached to the inner wall of the manhole.
A third type of observation well is manufactured by the applicant, The Boles Company, Inc. under the name Boles Test and Retrieval Sump.RTM.. In this observation well, a ribbed perforate steel screen, casing or liner being 2 to 12 inches in diameter is placed below the concrete. A unsealed plastic cap may be installed over the screen. A metal manhole with a metal manhole cover is set in a small raised area of the concrete. The manhole is not attached to the screen.
Seasonal fluctuations in the ground surface, particularly those associated with freezing and thawing, place great stress on the screen of an observation well which is permanently attached to the concrete. These wells are prone to having the screen separate from the concrete. The screen and the concrete are also subject to cracking and breaking.
In order that samples taken from an observation well are an accurate indication of the condition of the ground water and to prevent the well from being a path for contamination, it is critical that contamination not enter the interior of the well screen from the surface. Surface spills that occur and accumulate above the surface of the concrete have a tendency to leak through the manhole cover and into the observation well. Such contamination, and in particular petroleum or hydrocarbon components may seep through cracks in the concrete seal rendering it ineffective, may contaminate the underground water supply and, more importantly, provide an inaccurate representation of the condition of the storage tank.
The API well uses concrete as the seal to prevent leakage between the manhole ring and the well screen. Concrete is extremely prone to cracking and the potential for cracking is exacerbated by the permanent attachment of the well screen to the manhole ring, providing extra stress during freezing and thawing of the surface.
The Shell well uses a screen with band clamps to provide sealing between the manhole ring and well screen. This is an ineffective seal.
The current Boles Company, Inc. screen installation does not provide for a sealing beneath the manhole cover to avoid the egress of contamination from the surface.
Prior art observation well designs do not have an effective design to contain surface contamination and do not accommodate the heaving due to freezing and thawing.
The present invention is directed to maximizing the service life of an observation well by allowing relative motion between the observation well screen and the well cover. The invention further provides a more effective sealing means to prevent the egress of surface contamination into the well.